Well, you can’t just kill someone and take their heart out and then show up at a hospital with it and say “I found a donor!”
I suppose you might find some shady doctor somewhere who might not worry about the provenance of an organ, but it’s not likely a shady doctor would be skilled enough to do a heart transplant. Plus there is the fact that a donor must be alive when the organ is removed. An organ from a dead person is useless for transplants.